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Discussion Starter #1
What I have is a T140V engine going in a 1970 T120R frame so the rear sprocket will be 46 teeth and the front sprocket will be 20. So what kind of milage do you think I'll get at around 70 mph? Also, what do you think, would it be necessary to increase the front sprocket to 21 teeth for the engine to cruise at 70 comfortably for an hour or an hour and a half?
I was figuring that my tank is 2.5 gallons capacity and if it only gets 40 miles to the gallon @ 70 mph then it looks like 100 miles on a tank.
However, if it's only going to get 20 mpg then it's only 50 miles. Do you think it's possible to get 50 mpg out of this set up with no windshield on flat pavement?
 

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These days with the vibration and the cold I need to stop,for a pee anyway
Might as well do it at a petrol station
 

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Mine would do about 60 mpg, UK gallons, if i sat at 70 mph.Not that i ever do constant speeds.One annual run with passenger over the moors yields 60 mpg easily.I dont think i could hold onto the bars for 1 hour at 70 mph.
I use a 47/20 combination and it is a good set up.
 

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While appropriate higher gearing could increase your mpg running on the flat at speed, the amount you are going to be talking about is very small in real terms. Lets say to keep the maths simple you were getting 50 mpg originally and you could achieve a 10% increase in fuel mileage on 2 gallons you will get 110 miles instead of 100 miles- not really a margin of safety and one that could quickly disappear with a head wind or even changes in temperature, inconsistent fuel etc.

If I were going to cruise regularly at 70 mph for the duration of time you mention, I'd consider raising the stock gearing, to be a little easier on motor by reducing the rpm, but with a small tank I don't think that will help your overall range that much. Based on my 750 Tiger, I would definitely go higher on the gearing while I had the opportunity, I'm at altitude of 4500-5000 feet most of the time and mine still feels under geared in 5th.

I agree with Triton that about 45 mpg on a US gallon is about the best you will regularly do with a T140 motor running in the best of tune. Rambo's 60 mpg on a UK gallon is 48 mpg on a US gallon. I think 50 mp(US)g is achievable, but I think that would be your fair weather favorable conditions mileage and wouldn't want to rely on getting it, if a long walk was going to be the result of running out.
 

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I don't know the US/UK gallon conversion factor, all I can tell you is that my old 1976 T140 had the standard 20T/47T sprockets and when set up properly to run without silencers and air filters (I'm not recommending the latter but I would certainly recommend the former, and this makes the biggest difference in my experience), 5000rpm was 80mph, and that was an easy cruising speed to maintain all day, when I was 20 and didn't have bad wrists. Fuel consumption never went below 50mpg (British gallons) and was generally around 55mpg. After fitting higher compression pistons and a T140D 2-1 exhaust (no silencer), fuel consumption improved to nearer 60mpg. My current TR7 has never dropped below 60mpg and will easily do 70mpg if ridden at a steady 70mph over a long distance. Efficiency is the key, ie: less throttle opening for the same power, and the easiest route to that is to free the breathing up by ditching the silencers.
 

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Discussion Starter #8
These days with the vibration and the cold I need to stop,for a pee anyway
Might as well do it at a petrol station
Ha, Ha, You're right. I'd probably need to take a piss too! Mostly I was just wondering what type of mileage I could expect from my set up and whether or not you guys would think I'd be over revving the engine at 70 mph for long distances. I was reading John A's thread on his T140 rebuild and saw he was putting a 21 tooth front sprocket on his engine and was starting to ask questions that I could see was leading off the subject of his thread so decided to start this separate thread.
So far it looks like I can expect around 40 to maybe 45 mpg according to Redhawk4 and Triton Thrasher. I really don't care about the mileage I just wanted to know what to expect and don't want to ruin the engine by cruising to long of a time at high rpm's.

Thanks for everybody's responses,
Gary
 

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I don't agree that reducing rpm is easier on the motor.
Yeah,like the discussion we had on Britbike about gasoline combustion speeds, sometimes yes and sometimes no...:D

My bikes are for sport so fuel mileage isn't too important so long as it's better than my Jeep. And I don't have speedos on my bikes.
 

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Just be aware that a 21 tooth needs a fiddle to fit as it will not fit through the cover.
If you don't have a conical rear hub altering the rear sprocket size is more straight forward than putting a 21T on the front.

Regarding higher gearing being easier on the motor, I agree it's not totally straight forward, for example lugging in a higher gear is worse than than higher rpm. However if the gearing can be pulled easily and it perhaps puts your desired cruising speed in an rpm range where the engine is relatively smoother that has to be beneficial IMO.
 

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Regarding higher gearing being easier on the motor, I agree it's not totally straight forward, for example lugging in a higher gear is worse than than higher rpm. However if the gearing can be pulled easily and it perhaps puts your desired cruising speed in an rpm range where the engine is relatively smoother that has to be beneficial IMO.
Absolutely. Apart from some trials bikes with rear sprockets almost as big as their wheel rims, a bike with any gearing can be lugged on one hand or over-revved on the other. This is one of many areas where that elusive quality, commonsense, proves invaluable.

Raising the gearing on a 1970s T140/TR7, combined with getting the engine running as efficiently as possible, makes a lot of sense as they are quite undergeared in standard condition (they are also not very efficient in standard condition).
 

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Hi Code, Good to hear from you again. Glad you were spared from fire damage... weren't you?
Regarding the bike, you'll be right at 40-50mpg. That is what me & John have been running. Cruising slower like 55mph will give you about 10mpg more. We've been doing some 200mi. rides on flats & in hill so have recent experience. Do 60miles rides every other week or so. We use Chevron 91 fuel.
Regarding the 21 do not put that on. You will not have the power to pull the hills like Hwy 50 in high gear. 20x46 will be just perfect. Trust me on this.
Thinking of gearing, remember the difference in the 4&5 speed is the lower gears. Hi gear & the next gear lower are the same. With the 5 speed gears 1,2,3 are lower, which makes the 20x46 a really nice feeling set up. Will give good power on take off, even 2 up. I ran my TR6C 20x46 for many thousands of miles with trips up the Sierras & up the valley so I have personal experience on that gearing. My current TR7RV has 20x47. It is slightly low. 20x46 would be better. 46 is not avail. for my rear hub. I've considered 21x47, however I ride 2 up often & that would keep me in 4th gear most of the time up to near 50mph. Keep in mind if you lug these bikes at all... on our fuel, they will ping bad & ruin pistons.
John's bonnie was 19x20. We switch bikes often & it really spins @ 70. This very day he is installing a 20t front with original 46 rear. Over next few weeks, we'll trade bikes & I can give 1st hand account on difference with his bike. Especially regarding 1st gear take off. I rode 2 up often with wife on the TR6C with good results up many steep hills in Berkeley CA when we lived there.
Changing subject... March 28, 2015 is British Bike show in San Jose, I will be there with bike. Weather permitting I will also do morning after ride on 29th. 10am start in Los Gatos. Approx. 100mi, ride. If at all possible see if you can come down for one or both days, if your bike is going then. There is trailer parking at some hotels & some trailer parking on streets in town also. Assuming you don't ride down.
Don
 

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What needs to be kept in mind is one tooth up on the front sprocket is something like three teeth down on the rear which is quite a big step.
Even one tooth on the rear makes a noticeable difference to how a bike performs and feels (and vibrates)

davy
 

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21/45 is pretty much the same as 20/43. It is a lot higher than standard pre-1980 (in 1980 Triumph raised standard gearing to 20/45), presumably in belated recognition of the fact that 20/47 is needlessly low. I don't find any problems with 21/45 on my Tiger, which will readily pull in top from 3500 to 6500 in no time (and although I live in Norfolk I've ridden it where there are proper hills as well!). I must admit that if I lived in mountainous country I'd probably lower it to 21/47 just to give it an easier time.
 

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Hi Code, Good to hear from you again. Glad you were spared from fire damage... weren't you?
Regarding the bike, you'll be right at 40-50mpg. That is what me & John have been running. Cruising slower like 55mph will give you about 10mpg more. We've been doing some 200mi. rides on flats & in hill so have recent experience. Do 60miles rides every other week or so. We use Chevron 91 fuel.
Regarding the 21 do not put that on. You will not have the power to pull the hills like Hwy 50 in high gear. 20x46 will be just perfect. Trust me on this.
Thinking of gearing, remember the difference in the 4&5 speed is the lower gears. Hi gear & the next gear lower are the same. With the 5 speed gears 1,2,3 are lower, which makes the 20x46 a really nice feeling set up. Will give good power on take off, even 2 up. I ran my TR6C 20x46 for many thousands of miles with trips up the Sierras & up the valley so I have personal experience on that gearing. My current TR7RV has 20x47. It is slightly low. 20x46 would be better. 46 is not avail. for my rear hub. I've considered 21x47, however I ride 2 up often & that would keep me in 4th gear most of the time up to near 50mph. Keep in mind if you lug these bikes at all... on our fuel, they will ping bad & ruin pistons.
John's bonnie was 19x20. We switch bikes often & it really spins @ 70. This very day he is installing a 20t front with original 46 rear. Over next few weeks, we'll trade bikes & I can give 1st hand account on difference with his bike. Especially regarding 1st gear take off. I rode 2 up often with wife on the TR6C with good results up many steep hills in Berkeley CA when we lived there.
Changing subject... March 28, 2015 is British Bike show in San Jose, I will be there with bike. Weather permitting I will also do morning after ride on 29th. 10am start in Los Gatos. Approx. 100mi, ride. If at all possible see if you can come down for one or both days, if your bike is going then. There is trailer parking at some hotels & some trailer parking on streets in town also. Assuming you don't ride down.
Don
Are you saying 4th gear on a 5 speed is the same as 3rd on a 4 speed?
 

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Hi Red, Yes, that is what my books show in the "internal transmission ratios". Top or high gear (4th or 5th) is 1:1 on both, the next gear down is 1.9:1 for both.
Someday I hope to trade bikes with a 21x47 to see how it works in various conditions. One of my concerns is I'll be in 4th all the time which puts wear and tear on trans gears, where 5th is direct drive & counter shaft & gears have no load on them at all. If I use 5th at lower speeds bike will lug.
No idea when it may be since I know no one with a bike like that. I'd also like to ride a dynamically balanced 750, or even feel one running at standstill to see how it feels. Again no one I know has one.
Maybe on this years show & morning after ride will meet some new folks. Last year overnight rain caused many to leave early & not ride next day. Rain stopped 7am & we were worried it wouldn't so I don't blame them.
Don
 

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The only information I have to hand shows the overall gear ratios and shows the 4th gear on a four speed to be about the same as 4th gear on a 5 speed.
 
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